1. Field of the Invention
The invention relates to a method for carrying out integrity tests of at least one filter element, arranged within a filter housing, with porous filter materials, in which means are provided for measuring the sound generated by a test fluid when it flows through the filter element.
2. Description of the Related Art
The invention furthermore relates to an apparatus for carrying out integrity tests of at least one filter element, arranged within a filter housing, with porous filter materials, in which means are provided for measuring the sound generated by a test fluid when it flows through the filter element.
WO 94/11721 A1 has disclosed an apparatus and a method for carrying out integrity tests of filter elements, arranged within a filter housing, with porous filter material. There, a filter element wetted by a wetting solution subdivides a housing interior into an inlet side, or an unfiltrate space, and an outlet side, or filtrate space. Both sides can be filled with a test fluid, a pressure difference being generated between the inlet side and outlet side such that the test fluid flows through the filter element, with sound generated in the process being recorded by a sensor or a microphone, which is arranged in the vicinity of the wetted filter element or in the lumen of an inlet pipe or an outlet pipe, and being routed to a signal-processing unit as an electrical signal. The signal-processing unit analyzes the signal and determines whether or not the filter element is integral.
A disadvantage of this is that a test is carried out in this case that requires a wetted filter. The utilized pressure difference, or the pressure provided for flowing through the filter element, must in this case be less than the pressure at which the wetting agent is driven out of the largest pore of a homogenously porous structure of the filter material.
Furthermore, EP 0 248 218 B1 likewise discloses a method and an apparatus for carrying out integrity tests, in which a filter element, for example a hollow-fiber membrane, is arranged in a housing interior. In this case, a so-called bubble point test is also carried out. To this end, the filter material must in this case also be wetted by a wetting liquid. As the gas-space pressure increases with time, the sound intensity in a housing interior filled with a liquid is measured by a microphone, with the gas-space pressure being determined at which a jump-like increase in the sound intensity can be determined.
Furthermore, WO 99/16538 A1 discloses an apparatus and a method for localizing defective filter elements within a plurality of filter elements arranged within a housing. All filter elements arranged in a housing are also wetted by a liquid in this case and are subjected to a gas pressure, with the presence of defective filter elements being identified acoustically during a diffusion test by means of microphones.
Furthermore, JP 2006-218372 A (abstract) has disclosed a method and an apparatus for carrying out integrity tests of membrane filters arranged within a filter housing. The membrane filters subdivide a membrane module or a filter housing into an unfiltrate space and a filtrate space. A flow of gas between the two chambers can be determined by means for measuring sound generated in the membrane module.
This also has the disadvantage that the filter elements appear to have to be wetted.
U.S. Pat. No. 6,666,070 B1 has disclosed a method and a device for identifying integrity defects in the form of leaks in a honeycomb structure of a filter for capturing and burning diesel exhaust-gas particles. The honeycomb structure of the filter comprises a plurality of parallel cell channels, which traverse the honeycomb structure from an inlet end to an outlet end. In order to carry out the integrity test, compressed air is introduced through the inlet end of the honeycomb structure. An acoustic sensor is arranged at the outlet end of the honeycomb structure, and it identifies an airflow at the outlet end.
A disadvantage here is that the filter has a porosity that does not permit the compressed air to pass through the internal walls as long as there are no leaks. The air blower utilized in the process has a pressure that should lie between 1 and 15 psi. The fluid pressure acting on the filter materials changes depending on the leaks present, even in the case of an air blower operating at a fixedly set value. A further disadvantage is that the filter has to be inserted into a special test housing for the test.
DE 694 09 736 T2 has disclosed a method for carrying out integrity tests of a filter element, arranged within a filter housing, with a porous filter material; in the process, a gas mixture is applied onto a surface of the filter or the membrane structure under pressure in order to prompt the gas mixture to migrate through the pores in the membrane structure and emerge from a second surface of the same.
A disadvantage here is that in order to generate an acoustic emission, components of the gas mixture or of the test fluid, after emerging from the second surface, have to be excited by a light beam at a wavelength that is strongly absorbed by the component of the gas mixture. A further disadvantage is that this test also requires a wetted filter.
U.S. Pat. No. 6,568,282 B1 has disclosed a method for carrying out integrity tests on a filter, which is embodied as a porous membrane spanned in a housing. In the method, a fluid, which at least in part cannot be mixed with the wetting fluid used for the membrane, is supplied to a first side of the wetted membrane at a pressure that is higher than a pressure on a second side of the membrane. In the process, a predetermined pressure difference is set in the membrane or filter housing, and the transmission speed of the test fluid is subsequently measured from the first side of the membrane to the second side of the membrane. The measured transmission speed is subsequently compared to a second, predetermined transmission speed in order to determine the integrity.
It is also disadvantageous here that it requires a wetting of the filter material. Measuring sound has not been disclosed in this document, and there is no point in doing so on account of the utilized measurement principle.
Thus, it is an object of the present invention to specify a method and an apparatus that do not have the aforementioned disadvantages and wherein, more particularly, it is possible to carry out the method with unwetted filter elements.